Model-based analysis of photoinitiated coating degradation

Model-based analysis of photoinitiated coating degradation

Søren Kiil

Technical University of Denmark

Department of Chemical and Biochemical Engineering

Kgs. Lyngby, Denmark

Kurt Wood, Arkema

Coatings Research at DTU Chemical Engineering

CHEC Research Center

Research Topics (heavy duty coatings)

• Antifouling coatings for ships

• Anticorrosive coatings (incl. weathering)

• Blade coatings for wind turbines

• Intumescent coatings (fire protection)

2 DTU Chemical Engineering, Technical University of Denmark

People

• 2 professors (faculty)

• 3-5 Ph.D. students

• 2-4 M.Sc. Students

• Collaborators from coatings industry

Motivation and topics covered

• To obtain a ”complete” picture of UV-induced coating degradation

• Improve knowledge on interlayer adhesion loss

– problem between epoxy base coat and PU top coat

– loss of adhesion after few days of UV exposure

• Contents of presentation


• Contents of presentation

- mathematical model of coating degradation

- mechanisms and phenomena included

- verification

- high Tg coatings with narrow oxidation zones (2 µm)

- low Tg coatings with wide oxidation zones (250 µm)

- effect of light stabilizers

- future work

Thermoset coating exposed to UV radiation and humidity

Top view:


unexposed

exposed

Gloss loss not considered

Model contents

• Photoinitiated oxidation reactions

• Oxygen permeability

• Water absorption and diffusion


• Reduction of crosslink density (erosion)

• Development of an intrafilm oxidation zone

• Effect of light stabilizers (UV absorber and HALS)

For details on model assumptions see Kiil (2012), JCT Res.

Chemical degradation mechanism

• Case study with crosslinked epoxy-amine coatings

• Closed-loop chemical mechanism


• Reactions not included

Branching

Termination (not important here)

Adjustable parameters

• Rate constants for reaction (1) and/or (2)

(always included)

• Diffusion coefficient of oxygen in oxidation zone

(only when diffusion plays a role)


(only when diffusion plays a role)

• Stable conversion of crosslinks at coating surface when erosion is initiated (if erosion takes place)

• Rate constant of radical scavenger

(only when present)

Simulations and experimental data

ATR-FTIR


Clear coat, 50 °C, RH=75 %, Tg > 100 °C, UV=480 W/m2

NIST integrating sphere

(data from Nguyen et al., COSI, 2010)

Simulation of oxidation zone development

Clear coat, 50 °C, RH=75 %,

Tg > 100 °C, UV=480 W/m2

Oxygen diffusion very important


Oxidation zone thickness of about 2 µmin agreement with independent measurements by Monney et al. (1997, 1998, 1999)

Effect of nano-pigments on mass loss

50 °C, RH=75 %

Tg > 100 °C, UV=480 W/m2

MWCNT has a strong effect on stable

surface conversion


Nano-SiO2 has no effect

(same parameters used as for clear coat)

Experimental data from

Nguyen et al., NIST, (2010)

Effect of relative humidity, RH (50 °°°°C), on erosion rate

Transmission IR


RH=75 % RH=9 %

Experimental data from Rezig et al., NIST, (2006) (Tg=123 °C)

Simulations and experimental data (Mailhot et al., France, 2005)

0

0.2

0.4

0.6

0.8

1

yC

C (su

rfac

e v

alu

e)

0

0.002

0.004

0.006

0.008

0.01

∆O

D (

IR)

Simulation of yCC

Experimental data (∆OD)

0

0.08

0.16

0.24

0.32

0.4

0.48

0.56

yC

O

0

0.2

0.4

0.6

0.8

1

∆O

D (

167

5 c

m-1

)

Simulation


30 hours

ATR-IR data Micro-FTIR data


Clear coat, 60 °C, Tg=-50 °C, RH and UV unknown, no light stabilizers

0 40 80 120

Time (hours)

0

0.2

0.4

0.6

yC

O (

su

rfa

ce

va

lue

)

0

0.004

0.008

0.012

0.016

∆O

D (

IR)

0 0

Simulation of yCO


0 100 200 300

Position in coating (µm)

0

0.1

0.2

0.3

0.4

0.5

yC

O

0

0.2

0.4

0.6

0.8

∆O

D (

1675

cm

-1)

0 0

Simulation


110 hours

Effect of UV absorber on absorption and oxidation depth

Clear coat, 60 °C, Tg=-50 °C, 30 hours

Experimental data from Mailhot et al. (2004)

Without UV absorber the oxidation zone

thickness was 250 µm

0.10

0.2

0.4

0.6

0.8

1

El/E

O

Simulation


No oxygen concentration profile

Absorption of UV radiation limits

oxidation zone thickness

0 50 100 150


0

0.1

0.2

0.3

0.4

0.5

0.6

yC

O

0.05

0.06

0.07

0.08

0.09

0.1

∆O

D (

1675 c

m-1)

Simulation


Effect of molar absorptivity of UV absorber and rate constant of radical scavenger

Clear coat, 60 °C, Tg=-50 °C, 30 hours,

UV absorber present

0

0.1

0.2

0.3

0.4

0.5

0.6

yC

O

αUVA=600 m2/mol

αUVA=400 m2/mol

αUVA=200 m2/mol


0 50 100 150


0

0.1

0.2

0.3

0.4

0.5

yC

O

0

kRS=0

kRS=0.2 (m3/mol) s-1

kRS=0.5 (m3/mol) s-1

2 2R NO R R NOR⋅ + ⋅ →

2( )

R RS R R NOr k C C⋅ ⋅ ⋅− =

Reaction of radical scavenger (HALS):

UVA present

Conclusions

• Mathematical model developed for photoinitiated coating degradation

• The relative importance of the different rate phenomena quantified

• Model requires calibration of adjustable parameters for practical use

- from laboratory data to service life prediction


- from laboratory data to service life prediction

• Model cannot predict gloss loss and speciation of photoproducts

Goldschmidt and Streitberger (2007)

Future work

• Extend model to cyclic accelerated testing

- effects of rain erosion, time of wetness, UV radiation, and temperature

• Extend model to more complex epoxy-amine formulations

- including other pigments and additives


• Extend model to other coatings/binders

- requires a closed-loop mechanism and data for model calibration

• Use model for simulation of weather scenarios of commercial interest

• Detailed experimental data series required!

Thank you for your attention…

Financial support by the Hempel Foundation is gratefully acknowledged

Contact:Søren Kiil, [email protected]


Goldschmidt and Streitberger (2007)

Model-based analysis of photoinitiated coating degradation

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